WO2014112523A1 - Decryption-service provision device, processing device, safety evaluation device, program, and recording medium - Google Patents

Abstract

A secret key of a semigroup for which calculation of the original digits thereof is difficult with respect to computational complexity, said secret key being in accordance with the ElGamal encryption system, is stored, information corresponding to encrypted text in accordance with the ElGamal encryption system is inputted, a secret key (s) is used to decrypt the information corresponding to the encrypted text in accordance with the ElGamal encryption system, and information corresponding to the decryption result of the encrypted text is obtained and outputted. Furthermore, the degree of ease or the degree of difficulty with respect to the computational complexity of calculating the original digits of the semigroup is determined, and the safety of a decryption-service provision device is evaluated on the basis of the determination result.

Description

Decoding service providing apparatus, processing apparatus, safety evaluation apparatus, program, and recording medium

The present invention relates to information security technology, and more particularly to key management technology for restricting the parties who can use encrypted information after decryption.

Currently, in order to decrypt information encrypted by a general technique, a method of directly calculating a plaintext from a ciphertext using a key for decrypting the cipher is adopted (for example, see Non-Patent Document 1).

However, with this method, once the key has been handed over to the party concerned, the situation changes, and when it is desired to prohibit the party from decrypting with the key, it is not possible to force prohibition of decryption of the encryption.

Therefore, instead of distributing the key to the parties concerned, a method is conceived in which the server device provides a decryption service in response to a request from the parties held inside the reliable server device. Such a method is called cloud key management type encryption. In this method, the key for decrypting the cipher is not passed directly to the user, so it is expected that the user can stop using the encrypted data by stopping the decryption service. ing.

When providing a decryption service with the key held internally, depending on the encryption method, the user can repeatedly use the decryption service to gain the ability to decrypt the cipher, and the user can encrypt even if the decryption service is stopped. There is a possibility that it is not possible to stop using the data. Such a cloud key management type encryption has a problem in security.

The present invention has been made in view of these points, and an object thereof is to provide a technique for improving the security of cloud key management type encryption.

A secret key conforming to the ElGamal cipher on the semi-group where calculation of the original order is difficult in terms of calculation is held, information corresponding to the ciphertext conforming to the ElGamal cipher is input, and the secret key s is The information corresponding to the ciphertext is decrypted in accordance with the ElGamal encryption method, and the information corresponding to the decryption result of the ciphertext is obtained and output.

By using the present invention, it is possible to suppress the user from acquiring the ability to decrypt the cipher by repeatedly using the decryption service, and it is possible to improve the security of the cloud key management type cipher.

FIG. 1 is a block diagram for explaining the configuration of the security system of the first embodiment. FIG. 2 is a flowchart for explaining the decryption service providing process of the first embodiment. FIG. 3 is a block diagram for explaining the configuration of the security system of the second embodiment. FIG. 4 is a flowchart for explaining the safety evaluation process of the second embodiment. FIG. 5 is a block diagram for explaining the configuration of the security system of the third embodiment. FIG. 6 is a block diagram for explaining the configuration of the processing apparatus of the third embodiment. FIG. 7 is a block diagram for explaining the configuration of the decryption service providing apparatus according to the third embodiment. FIG. 8 is a flowchart for explaining the processing of the processing apparatus of the third embodiment. FIG. 9 is a flowchart for explaining the processing of the decryption service providing apparatus according to the third embodiment.

An embodiment of the present invention will be described.
<Principle>
The principle common to each embodiment will be described. In each embodiment, a key is held in a reliable decryption service providing apparatus, and the decryption service providing apparatus provides a decryption service in response to a request from a related party (cloud key management type encryption). However, a half group (for example, a commutative semigroup or a finite commutative semigroup) H in which calculation of the original order is difficult in terms of calculation is used, and decryption employing ElGamal encryption on the semigroup H as an encryption method. Use services. The reason why the decryption service is secure will be described below.

[ElGamal encryption method]
Next, the ElGamal encryption on the semigroup H will be described. The ElGamal cipher is an encryption scheme in which a reversible element of a semigroup H is gεH, and a secret key sεZ _q selected at random is a public key y = g ^−s εH. The ciphertext of g is given by (c ₁ , c ₂ ) = (m · y ^r , g ^r ) ∈H ² . rεZ _q is a random number determined in the encryption process, Z _q is a residue group by q, q is the order of a semigroup H (a positive integer, for example, a prime number), βεH is β This represents an element of the semigroup H, and “·” represents an operation defined in the semigroup H. In order to decrypt the ciphertext (c ₁ , c ₂ ), m ′ = c ₁ · c ₂ ^s εH may be calculated using the secret key s.

The decryption service device using the ElGamal cipher securely holds the secret key s and decrypts the ciphertext (c ₁ , c ₂ ) each time the user inputs the ciphertext (c ₁ , c ₂ ). The decoding result m ′ is output. At this time, the decryption service device determines whether or not to perform the decryption service to the user by using the authentication database that identifies the user by the authentication means and stores the user who can receive the decryption service properly. May output the decoding result.

[Attack on decryption service device using ElGamal encryption]
A polynomial time probabilistic algorithm B, which is an attacker trying to extract the secret key s using a decryption service device using ElGamal encryption, is formulated. For the given public key y = g- ^s , B attacks as follows.
1. B generates ciphertext c (i) = (c ₁ (i), c ₂ (i)) for i = 1, 2, 3,..., U (where u is a positive integer), and decrypts each. Input to the service device to obtain the decryption result w (i) = c ₁ (i) · c ₂ (i) ^s .
2. B is calculated using the information of c (i) and w (i), and outputs α∈Z _q where y = g ^−α .

Suppose B exists that succeeds in the attack. An arbitrary polynomial time probabilistic algorithm A that is an attacker can calculate the order of the element g of the semigroup H using B by the following method. Here, k is a positive integer security parameter.
1. A performs the following for i = 1, 2, 3,..., 3k to obtain β _i .
(A) Assuming that the element of the half group H is expressed by L bits at most, A selects a bit string λ that is sufficiently longer than L, and uses λ as a binary number as a secret key s.
(B) A calculates y = g ^−s and uses it as the public key.
(C) A uses B to obtain ^{α such} that y = g ^−α . If there is a decryption request from B on the way, A decrypts it using s and gives it to B.
(D) Let A be β _i = s−α.
2. A calculates the greatest common divisor of β ₁ , β ₂ ,..., Β _3k .

A outputs the order n of the element g except for a negligible probability of at most O (2 ^−k ) (O-notation). The reason will be described below. Since y = g ^−s = g ^−α , g ^βi = 1 for each i (the superscript βi represents β _i ). Therefore, β _i is an integral multiple of the order n of the element g of the half group H. Since s is chosen at random, integer _{r i} to be the β _i = nr _i is random. The probability that the greatest common divisor of r ₁ , r ₂ ,..., r _3k is not 1 can be evaluated using the Riemann zeta function and proves that it is at most O (2 ^−k ) or less. (This proof can be found, for example, in the following literature: Takeshi Yamamoto and Tetsutaro Kobayashi, “On Self-correction for Homomorphisms, Arithmetic Applications, SCIS2010) Therefore, the greatest common divisor of β ₁ , β ₂ ,..., Β _3k is the order n of the element g. Thus, if the above-described attack on the decryption service device using the ElGamal encryption is successful, the order of the element g of the semigroup H can be calculated. If this kinematic pair is also true and it is difficult to calculate the order of the element g of the semigroup H, it is difficult to perform the above-described attack on the decryption service device using the ElGamal encryption.

The following are examples of semigroups where the original order is difficult to calculate. Consider the remainder ring R = Z / NZ, where N is a number that is difficult to prime. It is known that computing the original order of the multiplicative group of R is as difficult as factoring N (this proof is for example written in: G. Miller , “Riemann's hypothesis and tests for primality,” Journal of Computer Systems Science, vol.13, pp.300-317, 1976.). That is, for example, a multiplicative group of a remainder ring R = Z / NZ modulo N can be used as a semi-group in which calculation of the original order is difficult. Another example is an elliptic curve on the ring R described above. A normal elliptic curve is formed on the body, but when the calculation rule is formally applied to the ring R, the coupling rule is established as in the case of the elliptic curve, and a semigroup is formed. When this is used as H, a random element extracted from H becomes a reversible element with a very high probability. When this is used as the element g, it has been found that calculating the order of the element g is as difficult as calculating the prime factorization of N (this detail is described, for example, in the following document) : Lenstra Jr., H. W., “Factoring integers with elliptic curves,” Annals of Mathematics 126 (3): 649-673, 1987.). That is, as a semigroup in which calculation of the original order is difficult, for example, a semigroup composed of points (rational points) on the elliptic curve on the ring R can be used. The semigroup in which the original order is difficult to calculate is, for example, a semigroup in which it is difficult to calculate the original order in order to solve the factorization problem. A certain calculation or problem is “difficult” means that the calculation result or solution cannot be obtained within the polynomial time. That is, “a semigroup in which calculation of the original order is difficult in terms of computational complexity” means, for example, a semigroup in which the original order cannot be calculated in polynomial time. The “semigroup in which the calculation of the original order is difficult in terms of computational complexity” may be one in which an inverse element exists or one in which an inverse element does not exist. Further, the “semigroup in which calculation of the original order is difficult in terms of calculation amount” may be, for example, a monoid (a semigroup having a unit element). “Polynomial time” means, for example, time (calculation time) that can be expressed by a polynomial of the size (length) of the secret key s. In other words, “polynomial time” means, for example, a time (calculation time) that can be expressed by an arbitrary polynomial for χ, where χ is the length (eg, bit length) of the secret key s. “Easy” means not difficult.

<First Embodiment>
Next, a first embodiment will be described. In the present embodiment, a decryption service is provided that uses a semigroup (commutative semigroup) H in which calculation of the order of the element g is difficult in terms of computational complexity and adopts an ElGamal cipher on the semigroup H as an encryption method. In this embodiment, “information corresponding to the ciphertext conforming to the ElGamal encryption scheme” is a ciphertext conforming to the ElGamal encryption scheme, and “information corresponding to the decryption result of the ciphertext” is the decryption result of the ciphertext.

[Constitution]
As illustrated in FIG. 1, the security system 1 of this embodiment includes an encryption device 11, a processing device 12, and a decryption service providing device 13. The encryption device 11 is configured to be able to provide information to the processing device 12 via a network, a portable recording medium, or the like. The processing device 12 and the decryption service providing device 13 are configured to exchange information with each other via a network, a portable recording medium, or the like.

The encryption device 11 includes a storage unit 111, an input unit 112, an encryption unit 113, and an output unit 114. The processing device 12 includes an input unit 121, a processing unit 122, an output unit 123, and an input unit 124. The decryption service providing apparatus 13 includes a storage unit 131, an input unit 132, a decryption unit 133, and an output unit 134. Each of the encryption device 11, the processing device 12, and the decryption service providing device 13 is configured such that a predetermined program is read into a general-purpose or dedicated computer including a CPU (central processing unit), a RAM (random-access memory), and the like. Special equipment. The encryption device 11, the processing device 12, and the decryption service providing device 13 execute each process under the control of each control unit (not shown). Data obtained by each unit is stored in a temporary memory (not shown), and is read by each unit as necessary.

[Parameter setting process]
In the parameter setting process of this embodiment, the semigroup H in which the calculation of the order of the element g is difficult in terms of computational complexity and the system parameters including the element g are set in the encryption device 11 and the decryption service device 13. A secret key sεZ _q according to the ElGamal encryption scheme on the semi-group H is selected at random and stored securely in the storage unit 131 of the decryption service providing apparatus 13. The public key y = g ^s ∈H conforming to ElGamal encryption scheme on the half-group H are generated and stored in the storage unit 111 of the encryption device 11.

[Encryption / decryption service provision processing]
As illustrated in FIG. 2, first, plaintext mεH is input to the input unit 112 of the encryption device 11 and sent to the encryption unit 113 (step S101). The encryption unit 113 uses the public key y stored in the storage unit 111 to encrypt the plaintext m according to the ElGamal encryption method on the semigroup H, and the ciphertext (c ₁ , c ₂ ) = (m · y ^r , g ^r ) εH ² is obtained and output (step S102). The output unit 114 outputs the ciphertext (c ₁ , c ₂ ).

The ciphertext (c ₁ , c ₂ ) is input to the processing device 12 via the input unit 121 and sent to the processing unit 122. (Step S104). The processing unit 122 generates decryption request information including the ciphertext (c ₁ , c ₂ ) and outputs it from the output unit 123 (step S105).

The decryption request information including the ciphertext (c ₁ , c ₂ ) is input to the input unit 132 of the decryption service providing device 13, and the ciphertext (c ₁ , c ₂ ) is sent to the decryption unit 133 (step S 106). The decryption unit 133 uses the secret key s stored in the storage unit 131 to decrypt the ciphertext (c ₁ , c ₂ ) in accordance with the ElGamal cryptosystem on the semi-group H and decrypts the decryption result m ′ = c _1. c ₂ ^s εH is obtained and output (step S107). The output unit 134 outputs response information including the decoding result m ′ (step S109). The response information is input to the processing device 12 and sent to the processing unit 122.

[Features of this form]
In this embodiment, the decryption service is provided by using the decryption service providing apparatus 13 that employs the ElGamal cipher on the semi-group H that is considered difficult to calculate the order of the element g. In this configuration, even if the user repeatedly uses the decryption operation service, it can be proved that it is difficult to obtain the secret key, and it is guaranteed that the secret key is not leaked in principle. It is considered that a generally secure encryption method cannot acquire a decryption capability without obtaining a secret key. Therefore, when performing the control to revoke permission after allowing the decryption operation service to the user, the user is encrypted after the use of the service is revoked no matter what behavior the user assumes. It can be said that the document cannot be decrypted.

Second Embodiment
Next, a second embodiment will be described. In this embodiment, the security of the decoding service is evaluated by determining the computational difficulty or ease of calculating the order of the element g of the semigroup H used for the decoding service. Thereby, the security of the decryption service can be evaluated. The security of such a decryption service may be evaluated at the time of setting the system parameter, or may be performed on a decryption service that has already been provided. Hereinafter, as an example, a mode in which the security of the decryption service is evaluated at the time of setting the system parameters, and the system parameters are reset when it is determined that the security is low will be described. In this embodiment, “information corresponding to the ciphertext conforming to the ElGamal encryption scheme” is a ciphertext conforming to the ElGamal encryption scheme, and “information corresponding to the decryption result of the ciphertext” is the decryption result of the ciphertext. Moreover, about the matter which is common in 1st Embodiment, description is abbreviate | omitted using the same reference number as 1st Embodiment.

[Constitution]
As illustrated in FIG. 3, the security system 2 of this embodiment includes an encryption device 11, a processing device 12, a decryption service providing device 13, a setting device 24, and a safety evaluation device 25. The setting device 24 is configured to be able to provide information to the encryption device 11 and the decryption service providing device 13 via a network or a portable recording medium. The setting device 24 and the safety evaluation device 25 are configured to exchange information with each other via a network, a portable recording medium, or the like. The setting device 24 and the safety evaluation device 25 are special devices configured by reading a predetermined program into a general-purpose or dedicated computer.

The safety evaluation device 25 includes a storage unit 251, an input unit 252, a determination unit 253, an evaluation unit 254, and an output unit 255. The safety evaluation device 25 executes each process under the control of a control unit (not shown). Data obtained by each unit is stored in a temporary memory (not shown), and is read by each unit as necessary.

[Parameter setting process]
In order to determine the computational difficulty or ease of calculating the order of the element g of the semigroup H in the storage unit 252 of the safety evaluation device 25 as pre-processing of the parameter setting processing of this embodiment. Store necessary information. Examples of such information include a semigroup that is considered difficult to calculate the original order and its original list, a semigroup that is considered easy to calculate the original order, and its The original list, or an algorithm for determining this difficulty or ease.

As illustrated in FIG. 4, in the parameter setting process, the setting device 24 generates a system parameter including a semigroup (for example, a commutative semigroup or a finite commutative semigroup) H and its element g (step S201). Information for specifying the generated semigroup H and element g is output (step S202).

Information for specifying the half group H and the element g is input to the input unit 252 of the safety evaluation device 25 and sent to the determination unit 253 (step S203). The determination unit 253 determines the difficulty or ease of calculating the order of the original g of the semigroup H using the information stored in the storage unit 251. For example, when the half group that is considered difficult to calculate the original rank of the half group and the list of the original group are stored in the storage unit 251, the determination unit 253 determines whether the half group H of the determination target It is determined whether the element g is included in the list, and if included, it is determined that it is difficult to calculate the order of the element g of the semigroup H. Alternatively, when the half group that is considered to be easy to calculate the original rank of the half group and the list of the original are stored in the storage unit 251, the determination unit 253 determines whether the half group H of the determination target It is determined whether the element g is included in the list, and if included, it is determined that it is easy to calculate the order of the element g of the semigroup H. When an algorithm for determining the difficulty or ease of calculating the original order of the half group is stored in the storage unit 251, the determination unit 253 uses this algorithm to determine the element of the half group H to be determined. It is determined whether it is difficult to calculate the order of g, or whether it is easy to calculate the order of the original g of the half group H to be determined. For example, if the discrete logarithm problem in the half group H is solved within a predetermined processing time using a number field sieving method or a function field sieving method, the order of the element g of the half group H can be easily calculated. (Step S204).

The determination result output from the determination unit 253 is input to the evaluation unit 254, and the evaluation unit 254 evaluates the safety of the decryption service providing apparatus 13 based on the determination result. That is, when the determination unit 253 determines that it is difficult to calculate the original order or it is not determined that the original order is easy to calculate, the evaluation unit 254 provides the decoding service. An evaluation result (acc) indicating that the safety of the device 13 is high is output (step S205). On the other hand, if the determination unit 253 does not determine that it is difficult to calculate the original order, or if it is determined that it is easy to calculate the original order, the evaluation unit 254 determines that the decoding service providing apparatus The evaluation result (rej) indicating that the safety of 13 is low is output (step S206).

The evaluation result output from the evaluation unit 254 is input to the setting device 24. The setting device 24 determines whether the evaluation result indicates that the safety is high (step S208). When the evaluation result indicates that the safety is low (rej), the process returns to step S201. If the evaluation result indicates that the safety is high (acc), the setting device 24 outputs the system parameters including the semigroup H and its element g, and sets the system parameters in the encryption device 11 and the decryption service providing device 13 (step) S209).

<Third Embodiment>
Next, a third embodiment will be described. In the present embodiment, a decoding service is performed using a self-correction technique using a semi-group in which calculation of the original order is difficult in terms of computational complexity. Details of the self-correction technique are disclosed in, for example, Reference 1 (International Publication WO / 2011/086992), Reference 2 (International Publication WO / 2012/077134), and the like. Below, it demonstrates centering on difference with 1st Embodiment.

<Configuration>
As illustrated in FIG. 5, the security system 3 according to the third embodiment includes, for example, the encryption device 11 described above, a processing device 321 that does not hold a secret key, and a secret key that conforms to the ElGamal encryption method described above. And the decryption service providing apparatus 332, the processing apparatus 321 requests the decryption service providing apparatus 332 to provide the decryption capability of the ciphertext, and the ciphertext is transmitted using the decryption capability provided from the decryption service providing apparatus 332. Is decrypted. The processing device 321 and the decryption service providing device 332 are configured to exchange information. For example, the processing device 321 and the decryption service providing device 332 can exchange information via a transmission line, a network, a portable recording medium, or the like.

As illustrated in FIG. 6, the processing device 321 according to the third embodiment includes, for example, an integer selection unit 2102, an input information providing unit 2104, a first calculation unit 2105, a first power calculation unit 2106, and a first list storage unit 2107. A second calculation unit 2108, a second power calculation unit 2109, a second list storage unit 2110, a determination unit 2111, a final output unit 2112, and a control unit 2113. Examples of the processing device 321 include a device having a calculation function and a storage function such as a card reader / writer device or a mobile phone, a CPU (central processing unit) or a RAM (random-access memory) loaded with a special program. It is a known or dedicated computer provided.

As illustrated in FIG. 7, the decryption service providing apparatus 332 according to the third embodiment includes, for example, a first output information calculation unit 2201 (decryption unit), a second output information calculation unit 2202 (decryption unit), and a key storage unit 2204. (Storage unit), control unit 2205, input unit 3132, and output unit 3134. The example of the decryption service providing apparatus 332 includes a tamper-resistant module such as an IC card or an IC chip, a device such as a mobile phone having a calculation function and a storage function, and a CPU or RAM into which a special program is read. Or a known or dedicated computer.

<Processing>
Next, the processing of this embodiment will be described. As a premise of processing, G and H are set to a semigroup (for example, a commutative semigroup or a finite commutative semigroup) in which calculation of the original order is difficult (for example, H = G × G, H = G), and f (x) is obtained by decrypting the ciphertext x, which is an element of the semigroup H, with a specific secret key s in accordance with the above-described ElGamal cryptosystem to obtain an element of the semigroup G It is assumed that the generation functions of the semigroups G and H are μ _g and μ _h , X ₁ and X ₂ are random variables having values in the semigroup G, the realization value of the random variable X ₁ is x ₁ , and the probability the actual values of the variables X ₂ and x _2. Each process of the processing device 321 is executed under the control of the control unit 2113, and each process of the decryption service providing device 332 is executed under the control of the control unit 2205.

As illustrated in FIG. 8, first, the integer selection unit 2102 of the processing device 321 (FIG. 6) selects integers a, b, a ′, and b ′ (step S2101). For example, a and b are relatively prime natural numbers. a and b are selected at random, for example. a ′ and b ′ may be selected randomly or may be selected so as to satisfy d = a′a + b′b. Information on at least part of the integers a and b is sent to the input information providing unit 2104, the first power calculation unit 2106, and the second power calculation unit 2109. Information on the integers a, b, a ′, and b ′ is sent to the final output unit 2112.

The control unit 1113 sets t = 1 (step S2102).

The input information providing unit 2104 receives the first input information τ ₁ and the second input information τ ₂ (in the ciphertext x) that are elements of the half group H corresponding to the input ciphertext x in accordance with the ElGamal encryption method. Corresponding information) is generated and output (step S2103). Preferably, the first input information τ ₁ and the second input information τ ₂ are information that disturbs the relationship with the ciphertext x. As a result, the processing device 321 can conceal the ciphertext x from the decryption service providing device 332. Preferably, the first input information τ ₁ of this embodiment further corresponds to the integer b selected by the integer selector 2102, and the second input information τ ₂ further corresponds to the integer a selected by the integer selector 2102. Thereby, the processing device 321 can evaluate the decoding capability provided from the decoding service providing device 332 with high accuracy. Specific examples of τ ₁ and τ ₂ are exemplified in Reference Documents 1 and 2, for example. For example, x = (c ₁ , c ₂ ), (V, W) is an element of the group H, f (V, W) = Y, and r ₄ to r ₇ are natural numbers of 0 or more. , and the a ^{_{τ 1 = (c 2 b W}} r4, c 1 b V r4 μ g r5), a ^{_{τ 2 = (c 2 a W}} r6, c 1 a V r6 μ g r7).

As illustrated in FIG. 9, the first input information τ ₁ is input to the input unit 3132 of the decryption service providing apparatus 332 (FIG. 7) and is input from there to the first output information calculation unit 2201. The second input information τ ₂ is input to the input unit 3132 and from there to the second output information calculation unit 2202 (step S2200).

The first output information calculation unit 2201 uses the first input information τ ₁ and the secret key s stored in the key storage unit 2204 to correctly calculate f (τ ₁ ) with a probability larger than a certain probability. The calculated result is defined as first output information z ₁ . That is, the first output information calculation unit 2201 can correctly calculate f (τ ₁ ) using the first input information τ ₁ and the secret key s stored in the key storage unit 2204, and the obtained calculation result Is the first output information z ₁ (step S2201). The second output information calculation unit 2202 uses the second input information τ ₂ and the secret key s stored in the key storage unit 1204 to correctly calculate f (τ ₂ ) with a probability greater than a certain probability. the calculation result as the second output information z _2. That is, the second output information calculation unit 2202 can correctly calculate f (τ ₂ ) using the second input information τ ₂ and the secret key s stored in the key storage unit 1204, and the obtained calculation result Is the second output information z ₂ (step S2202). That is, the first output information calculation unit 2201 and the second output information calculation unit 2202 output a calculation result including an intentional or unintentional error. In other words, also when the calculation result in the first output information calculation section 2201 is not f Some cases of _{(τ 1) f (τ 1} ), the calculation result of the second output information calculation unit 2202 f (tau ₂ ) and not f (τ ₂ ). “Computable” means that the calculation can be performed with a probability higher than the probability that cannot be ignored. The probability that cannot be ignored is a probability of 1 / F (k) or more, assuming that a polynomial that is a broad monotone function for the security parameter k is a polynomial F (k).

The first output information calculation unit 2201 outputs the first output information z ₁ , the second output information calculation unit 2202 outputs the second output information z ₂ , and the output unit 3134 outputs the first output information z ₁ and the second output information z ₁ . Output information z ₂ (information corresponding to the decryption result of the ciphertext x) is output (step S2203).

Returning to FIG. 8, the first output information z ₁ is input to the first calculation unit 2105 of the processing device 321 (FIG. 6), and the second output information z ₂ is input to the second calculation unit 2108. The first output information z ₁ and the second output information z ₂ correspond to the decryption capability given from the decryption service providing apparatus 332 to the processing apparatus 321 (step S2104).

First calculation unit 2105, generates a calculation result ^{u = f (x) b x} 1 from the first output information z _{1 (e.g.,} satisfy _{^{u = z 1 Y -r4 μ g}} -r5). The calculation result u is sent to the first power calculation unit 2106 (step S2105).

The first power calculating unit 2106 calculates the u '= u ^a. A set (u, u ′) of the calculation result u and u ′ calculated based on the calculation result is stored in the first list storage unit 2107 (step S2106).

The determination unit 2111 determines that u ′ = v ′ in the set (u, u ′) stored in the first list storage unit 2107 and the set (v, v ′) stored in the second list storage unit 2110. It is determined whether or not there is any (step S2107). If the set (v, v ′) is not stored in the second list storage unit 2110, the process of the next step S2108 is performed without performing the process of step S2107. If there is one that satisfies u ′ = v ′, the process proceeds to step S2115. If there is no u ′ = v ′, the process advances to step S2108.

In step S2108, the second calculation unit 2108, the calculation result from the second output information z ₂ v = f (x) generates a ^a x _{2 (e.g.,} satisfy _{^{v = z 2 Y -r6 μ g}} -r7). The calculation result v is sent to the second power calculation unit 2109 (step S2108).

The second power calculation unit 2109 calculates v ′ = v ^b . A set (v, v ′) of the calculation result v and v ′ calculated based on the calculation result is stored in the second list storage unit 2110 (step S2109).

The determination unit 2111 determines that u ′ = v ′ in the set (u, u ′) stored in the first list storage unit 2107 and the set (v, v ′) stored in the second list storage unit 2110. It is determined whether or not there is any (step S2110). If there is one that satisfies u ′ = v ′, the process proceeds to step S2115. If there is no u ′ = v ′, the process advances to step S2111.

In step S2111, the control unit 2113 determines whether t = T _max (step S2111). T _max is a predetermined natural number. If t = _Tmax , the control unit 2113 outputs information indicating that the calculation could not be performed, for example, the symbol “⊥” (step S2113), and ends the process. If not t = _Tmax , the control unit 2113 increments t by 1, that is, sets t = t + 1 (step S2112), and returns to step S2103.

In step S2115, the final output unit 2112 satisfies u ′ and v ′ corresponding to u ′ and v ′ where u ′ = v ′, and a ′, b satisfying d = a′a + b′b (for example, d = 1). Using “and d”, (u ^{b ′} v ^{a ′} ) ^{1 / d} is calculated and output (step S2115). That is, the final output unit 2112 outputs (u ^{b ′} v ^{a ′} ) ^{1 / d} for d = a′a + b′b. For example, when d = 1, the final output unit 2112 outputs u ^{b ′} v ^{a ′} for 1 = a′a + b′b. The output (u ^{b ′} v ^{a ′} ) ^{1 / d} becomes a decryption result f (x) of the ciphertext x with a high probability (see the references 1 and 2 for the reason, for example). Further, the above-described processing may be repeated a plurality of times, and the most frequently used value among the values obtained in step S2115 may be used as the decoding result.

<Modification of Third Embodiment>
One of the integers a and b may be a constant such as 1, and at least one of the integers a ′ and b ′ may be a constant. Some of the integers a, b, a ′, and b ′ may be constants, and some processing units and steps may be omitted. For example, when b is a constant 1, it is not necessary to select the integer b in step S2101, v ′ = v, and the second power calculation unit 2109 and step S2109 are not required. Alternatively, d may be a constant or a random number, and integers a, b, a ′, and b ′ that satisfy d = a′a + b′b may be selected in step S2101. Alternatively, d = a′a + b′b may be calculated in step S2115. Alternatively, in step S2101, the integer selection unit 2112 may select integers a, b, a ′, and b ′ and calculate d = a′a + b′b. Alternatively, the integer selection unit 2102 may calculate d = a′a + b′b at any time after step S2102 and before step S2115. The final calculation unit 2112 may output (u ^{b ′} v ^{a ′} ) ^{1 / d} when d ≠ 0. If d = 0, the process may end with an error. If d = 0, at least a part of the integers a, b, a ′, b ′ is reselected in step S2101 and the process is repeated. Good. Also, a ′ and b ′ may be selected at any time point before a ′ and b ′ are required for the calculation.

<Other variations, etc.>
The present invention is not limited to the embodiment described above. For example, the self-correction technique group of each embodiment such as JP2012-237881, JP2012-220835, JP2012-220814, and JP2012-151756 may be replaced with the above-mentioned semi-group. Good. For example, the security system may further include a history storage device that stores a list including a list of users who have been provided with the decryption service by the decryption service providing device 13 and a decryption result. In the configuration of each embodiment, it is difficult to obtain a secret key even if the user repeatedly uses the decryption operation service. Therefore, all the decryption results obtained using the secret key are those output from the decryption service providing apparatus 13. Therefore, for example, when a decoding result leaks, information for tracking the leaked path can be obtained by verifying a list or log of users stored in the history storage device. In the embodiments described above order of the Hangun H were the same q and order of the Z _q. However, the order of the half group H may be larger or smaller than q. Depending on the encryption method, the half group H may not be commutative.

The various processes described above are not only executed in time series in accordance with the description, but may also be executed in parallel or individually as required by the processing capability of the apparatus that executes the processes. Needless to say, other modifications are possible without departing from the spirit of the present invention.

When the above-described configuration is realized by a computer having a hardware processor such as a CPU or a memory such as a RAM, the processing contents of functions that each device should have are described by a program. By executing this program on a computer, the above processing functions are realized on the computer. The program describing the processing contents can be recorded on a computer-readable recording medium. An example of a computer-readable recording medium is a non-transitory recording medium. Examples of such a recording medium are a magnetic recording device, an optical disk, a magneto-optical recording medium, a semiconductor memory, and the like.

This program is distributed, for example, by selling, transferring, or lending a portable recording medium such as a DVD or CD-ROM in which the program is recorded. Furthermore, the program may be distributed by storing the program in a storage device of the server computer and transferring the program from the server computer to another computer via a network.

For example, a computer that executes such a program first stores a program recorded on a portable recording medium or a program transferred from a server computer in its own storage device. When executing the process, this computer reads a program stored in its own recording device and executes a process according to the read program. As another execution form of the program, the computer may read the program directly from the portable recording medium and execute processing according to the program, and each time the program is transferred from the server computer to the computer. The processing according to the received program may be executed sequentially. The above-described processing may be executed by a so-called ASP (Application Service Provider) type service that does not transfer a program from the server computer to the computer but implements a processing function only by the execution instruction and result acquisition. Good.

In the above embodiment, the processing functions of the apparatus are realized by executing a predetermined program on a computer, but at least a part of these processing functions may be realized by dedicated or general-purpose hardware.

1, 2, 3 Security system 11 Encryption device 12,321 Processing device 13,332 Decryption service providing device 24 Setting device 25 Safety evaluation device

Claims (14)

1. A storage unit that holds a secret key in accordance with the ElGamal cryptosystem on the semi-group where calculation of the original order is computationally difficult,
   An input unit for inputting information corresponding to ciphertext in accordance with the ElGamal encryption method;
   Using the secret key, decrypting information corresponding to the ciphertext in accordance with the ElGamal encryption method, and obtaining a information corresponding to the decryption result of the ciphertext;
   An output unit for outputting information corresponding to the decoding result;
   An apparatus for providing a decryption service.
2. The decryption service providing apparatus according to claim 1,
   The secret key is s, the semigroup is H, the ciphertext includes elements c ₁ and c ₂ εH of the semigroup H, and the decryption result is c ₁ · c ₂ ^s εH Decoding service providing device.
3. The decryption service providing apparatus according to claim 1 or 2,
   In order to obtain the element of the half group G by decrypting the ciphertext x which is an element of the half group H according to the ElGamal cryptosystem with a specific secret key, where G and H are the half group, and f (x) is the ElGamal cryptosystem , X ₁ , X ₂ is a random variable having a value in the semigroup G, x ₁ is a real value of the random variable X ₁ , x ₂ is a real value of the random variable X ₂ , information corresponding to the ciphertext Are the first input information τ ₁ and the second input information τ ₂ that are elements of the half group H,
   The decoding unit
   F (τ ₁ ) can be correctly calculated using the first input information τ ₁ , and the obtained calculation result is defined as first output information z ₁ .
   F (τ ₂ ) can be correctly calculated using the second input information τ ₂ , and the obtained calculation result is set as second output information z ₂ .
   The decryption service providing apparatus, wherein the information corresponding to the decryption result includes the first output information z ₁ and the first output information z ₁ .
4. Decipher the ciphertext x that is an element of the half group H according to the ElGamal cryptosystem with a specific secret key, where G and H are computationally difficult to calculate the original order, and f (x) A decoding function for obtaining elements of the semigroup G, X ₁ and X ₂ are random variables having values in the semigroup G, x ₁ is an actual value of the random variable X ₁ , and x ₂ is a random variable X ₂ Realized values a, b, a ′, b ′ are integers, d ≠ 0, d = a′a + b′b,
   A first calculation unit for generating a calculation result u = f (x) ^b x ₁ ;
   A second calculation unit for generating ^a calculation result v = f (x) ^a x ₂ ;
   A final calculation unit that outputs (u ^{b ′} v ^{a ′} ) ^{1 / d} when the calculation results u and v satisfy u ^a = v ^b ;
   A processing apparatus.
5. Holds a secret key conforming to the ElGamal cryptosystem on the semi-group, and information corresponding to the ciphertext conforming to the ElGamal cryptosystem is input, and the ciphertext is conformed to the ElGamal cryptosystem using the secret key. Difficulty in calculating the original order of the half group used in the decryption service providing apparatus that outputs information corresponding to the decryption result of the ciphertext obtained by decrypting the corresponding information A determination unit for determining the performance or ease;
   An evaluation unit that evaluates the safety of the decryption service providing device based on a determination result in the determination unit;
   A safety evaluation device.
6. The safety evaluation device according to claim 5,
   The secret key is s, the semigroup is H, the ciphertext includes elements c ₁ and c ₂ εH of the semigroup H, and the decryption result is c ₁ · c ₂ ^s εH ,
   A safety evaluation device characterized by that.
7. An input step in which information corresponding to ciphertext conforming to the ElGamal cryptosystem on the semi-group where calculation of the original order is computationally difficult is input,
   Using a secret key conforming to the ElGamal encryption method, decrypting information corresponding to the ciphertext according to the ElGamal encryption method, and obtaining a information corresponding to a decryption result of the ciphertext;
   An output step of outputting information corresponding to the decoding result;
   A method for providing a decryption service.
8. The decryption service providing method according to claim 7, comprising:
   The secret key is s, the semigroup is H, the ciphertext includes elements c ₁ and c ₂ εH of the semigroup H, and the decryption result is c ₁ · c ₂ ^s εH Decoding service providing method.
9. The decryption service providing method according to claim 7 or 8, comprising:
   In order to obtain the element of the half group G by decrypting the ciphertext x which is an element of the half group H according to the ElGamal cryptosystem with a specific secret key, where G and H are the half group, and f (x) is the ElGamal cryptosystem , X ₁ , X ₂ is a random variable having a value in the semigroup G, x ₁ is a real value of the random variable X ₁ , x ₂ is a real value of the random variable X ₂ , information corresponding to the ciphertext Are the first input information τ ₁ and the second input information τ ₂ that are elements of the half group H,
   The decoding step includes
   F (τ ₁ ) can be correctly calculated using the first input information τ ₁ , and the obtained calculation result is defined as first output information z ₁ .
   F (τ ₂ ) can be correctly calculated using the second input information τ ₂ , and the obtained calculation result is set as second output information z ₂ .
   The decoding service providing method, wherein the information corresponding to the decoding result includes the first output information z ₁ and the first output information z ₁ .
10. Decipher the ciphertext x that is an element of the half group H according to the ElGamal cryptosystem with a specific secret key, where G and H are computationally difficult to calculate the original order, and f (x) A decoding function for obtaining elements of the semigroup G, X ₁ and X ₂ are random variables having values in the semigroup G, x ₁ is an actual value of the random variable X ₁ , and x ₂ is a random variable X ₂ Realized values a, b, a ′, b ′ are integers, d ≠ 0, d = a′a + b′b,
    A first calculation step for generating a calculation result u = f (x) ^b x ₁ ;
    A second calculation step for generating ^a calculation result v = f (x) ^a x ₂ ;
    A final calculation step of outputting (u ^{b ′} v ^{a ′} ) ^{1 / d} when the calculation results u and v satisfy u ^a = v ^b ;
    A processing method comprising:
11. A secret key conforming to the ElGamal encryption scheme on the semi-group is held, and an encrypted text conforming to the ElGamal encryption scheme is input, and the ciphertext is decrypted according to the ElGamal encryption scheme using the secret key. A determination step for determining the computational difficulty or ease of calculating the original order of the semi-group used in the decoding service providing method for outputting the decoded decoding result;
    An evaluation step for evaluating the security of the decryption service providing method based on the determination result in the determination step;
    A safety evaluation method.
12. The safety evaluation method according to claim 11, comprising:
    The secret key is s, the semigroup is H, the ciphertext includes elements c ₁ and c ₂ εH of the semigroup H, and the decryption result is c ₁ · c ₂ ^s εH ,
    A safety evaluation method characterized by that.
13. A program for causing a computer to function as the decryption service providing device according to any one of claims 1 to 3, the processing device according to claim 4, or the decryption service providing device according to claim 5 or 6.
14. A computer-readable recording medium storing a program for causing a computer to function as the decryption service providing device according to claim 1, the processing device according to claim 4, or the decryption service providing device according to claim 5 or 6. .

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